XMLHttpRequest to Post HTML Form

Question

Current Setup

I have an HTML form like so.

<form id="demo-form" action="post-handler.php" method="POST">
   <input type="text" name="name" value="previousValue"/>
   <button type="submit" name="action" value="dosomething">Update</button>
</form>

I may have many of these forms on a page.

My Question

How do I submit this form asynchronously and not get redirected or refresh the page? I know how to use XMLHttpRequest. The issue I have is retrieving the data from the HTML in javascript to then put into a post request string. Here is the method I'm currently using for my zXMLHttpRequest`'s.

function getHttpRequest() {
    var xmlhttp;
    if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
    } else {// code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    
    return xmlhttp;
}

function demoRequest() {
       var request = getHttpRequest();
       request.onreadystatechange=function() {
             if (request.readyState == 4 && request.status == 200) {
                   console.log("Response Received");
             }
       }
       request.open("POST","post-handler.php",true);
       request.setRequestHeader("Content-type","application/x-www-form-urlencoded");
       request.send("action=dosomething");
}

So for example, say the javascript method demoRequest() was called when the form's submit button was clicked, how do I access the form's values from this method to then add it to the XMLHttpRequest?

EDIT

Trying to implement a solution from an answer below I have modified my form like so.

<form id="demo-form">
       <input type="text" name="name" value="previousValue"/>
       <button type="submit" name="action" value="dosomething" onClick="demoRequest()">Update</button>
</form>

However, on clicking the button, it's still trying to redirect me (to where I'm unsure) and my method isn't called?

Button Event Listener

document.getElementById('updateBtn').addEventListener('click', function (evt) {
                                evt.preventDefault();
                                
                                // Do something
                                updateProperties();
                                
                                return false;
                            });

Answer 1

The POST string format is the following:

name=value&name2=value2&name3=value3

So you have to grab all names, their values and put them into that format. You can either iterate all input elements or get specific ones by calling `document.getElementById()`.

Warning: You have to use encodeURIComponent() for all names and especially for the values so that possible & contained in the strings do not break the format.

Example:

var input = document.getElementById("my-input-id");
var inputData = encodeURIComponent(input.value);

request.send("action=dosomething&" + input.name + "=" + inputData);

---

Another far simpler option would be to use FormData objects. Such an object can hold name and value pairs.

Luckily, we can construct a FormData object from an existing form and we can send it it directly to XMLHttpRequest's method send():

var formData = new FormData( document.getElementById("my-form-id") );
xhr.send(formData);

Answer 2

I have had this problem too, I think.

I have a input element with a button. The onclick method of the button uses XMLHttpRequest to POST a request to the server, all coded in the JavaScript.

When I wrapped the input and the button in a form the form's action property was used. The button was not type=submit which form my reading of HTML standard (https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#attributes-for-form-submission) it should be.

But I solved it by overriding the form.onsubmit method like so:

form.onsubmit = function(E){return false;}

I was using FireFox developer edition and chromium 38.0.2125.111 Ubuntu 14.04 (290379) (64-bit).

Answer 3

The ComFreek's answer is correct but a complete example is missing.

Therefore I have wrote an extremely simplified working snippet:

<!DOCTYPE html>
<html>
<head>
<meta http-equiv="X-UA-Compatible" content="IE=Edge, chrome=1"/>

<script>
"use strict";
function submitForm(oFormElement)
{
  var xhr = new XMLHttpRequest();
  xhr.onload = function(){ alert(xhr.responseText); }
  xhr.open(oFormElement.method, oFormElement.getAttribute("action"));
  xhr.send(new FormData(oFormElement));
  return false;
}
</script>
</head>

<body>
<form method="POST"
      action="post-handler.php"
      onsubmit="return submitForm(this);" >
   <input type="text"   value="previousValue" name="name"/>
   <input type="submit" value="Update"/>
</form>
</body>
</html>

This snippet is basic and cannot use GET. I have been inspired from the excellent Mozilla Documentation. Have a deeper read of this MDN documentation to do more. See also this answer using formAction.

Answer 4

function postt(){
    var http = new XMLHttpRequest();
    var y = document.getElementById("user").value;
    var z = document.getElementById("pass").value;
    var postdata= "username=y&password=z"; //Probably need the escape method for values here, like you did

    http.open("POST", "chat.php", true);

    //Send the proper header information along with the request
    http.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    http.setRequestHeader("Content-length", postdata.length);

    http.onreadystatechange = function() {//Call a function when the state changes.
        if(http.readyState == 4 && http.status == 200) {
            alert(http.responseText);
        }
    }
    http.send(postdata);
}

---

how can I post the values of y and z here from the form

Answer 5

With pure Javascript, you just want something like:

var val = document.getElementById("inputFieldID").value;

You want to compose a data object that has key-value pairs, kind of like

name=John&lastName=Smith&age=3

Then send it with request.send("name=John&lastName=Smith&age=3");

Answer 6

By the way I have used the following code to submit form in ajax request.

 $('form[id=demo-form]').submit(function (event) {

    if (request) {
        request.abort();
    }
    // setup some local variables
    var $form = $(this);

    // let's select and cache all the fields
    var $inputs = $form.find("input, select, button, textarea");


    // serialize the data in the form
    var serializedData = $form.serialize();


    // fire off the request to specific url

    var request = $.ajax({
        url : "URL TO POST FORM",
        type: "post",
        data: serializedData
    });
    // callback handler that will be called on success
    request.done(function (response, textStatus, jqXHR){


    });

    // callback handler that will be called on failure
    request.fail(function (jqXHR, textStatus, errorThrown){

    });

    // callback handler that will be called regardless
    // if the request failed or succeeded
    request.always(function () {
        // reenable the inputs

    });

    // prevent default posting of form
    event.preventDefault();
});